The steam generators of such power stations generally comprise a bundle constituted by a very large number of small diameter tubes folded into U-shapes and fixed at each of their ends by being mounted in a very thick perforated plate. The portion of the steam generator situated beneath the perforated plate constitutes a water box in a first portion of which "primary" water under pressure coming from the reactor vessel is distributed amongst the tubes of the bundle. After flowing along the tubes, the water is recovered in a second portion thereof and is returned via a pipe of the reactor primary circuit to the vessel which contains the reactor core which is constituted by fuel assemblies for heating the pressurized water. The steam generator feed water comes into contact with the outside surfaces of the tubes in the bundle in a portion of the steam generator which is situated above the perforated plate, and is transformed into steam, which steam is delivered to the turbine associated with the nuclear reactor.
The walls of the tubes in the bundle thus constitute a barrier between the pressurized water constituting the primary fluid of the reactor and the feed water constituting the secondary fluid. The primary fluid which comes into contact with the fuel assemblies and with other structures inside the reactor vessel contains radioactive substances to a greater or lesser extent. It is therefore appropriate to avoid any direct contact between the primary fluid and the secondary fluid, since contamination of the secondary fluid could have highly damaging consequences. To make this possible, it is necessary to avoid leaks occurring through the walls of the tubes in the steam generator bundle of tubes, and in the event that such leaks do occur, it is necessary to repair tubes having leaky walls effectively and as quickly as possible.
Such repair work is generally required during the normal lifetime of the reactor. Stresses of thermal or mechanical origin or due to corrosion to which the tubes in the bundle are subject may give rise to cracks through the walls of the tubes in the bundle either in the primary side thereof or the secondary side. Repairs are performed during reactor downtime for maintenance purposes.
A known method of repairing the tubes in the bundle of a steam generator consists in bushing said tubes where they are leaking. A bush having an outside diameter which is slightly smaller than the inside diameter of the tube to be repaired and of sufficient length to cover the zone including a leak is inserted into the tube via the inlet face of the perforated plate at which the ends of the tubes are flush. The bush is placed so as to be flush with the inlet face of the perforated plate when inserted in a tube situated in the middle of the perforated plate, or so as to be halfway through the perforated plate when situated in a tube closer to the periphery thereof. The bush is then fixed inside the tube so as to provide two watertight connections between the bush and the tube, one on either side of the zone including the leak.
All of these operations require action to be performed from inside the water box of the steam generator, i.e. from inside a portion of the steam generator which has been in contact with primary fluid containing radioactive substances while the reactor was in operation, and this means that a human operator is subjected to a large dose of radioactivity.
Proposals have therefore been made to provide remotely controlled automatic devices capable of performing at least some of the operations required for bushing the tubes, in particular the bush can be fixed to the tube by being welded thereto at each of its ends.
U.S. Pat. No. 4,694,136 describes a device for performing such welding. This device comprises an optical weld head installed in the tube and a power laser situated outside the water box of the steam generator. The laser beam is transported by open air optical means from the laser situated outside the water box of the steam generator and the weld head.
This device requires a considerable amount of equipment to be implemented and installed in the vicinity of the steam generator including all the necessary supplies of fluids (electricity supply, cooling water supply). The power laser must be installed very close to the inspection hatch of the water box, which region is subjected to a level of radiation which is almost as high as that inside the water box. Attempts have been made to move the laser away from the inspection hatch, but this distance cannot be made very great without complicating aiming problems which rapidly become unsurmountable. Further, the weld head used suffers from a drawback related to the fact that the focal length is maintained constant and equal to the firing distance by an abutment having a ball and a spring bearing thereagainst: this system gives rise to friction, and to jolts and vibrations which reduce the quality of the welding.
In the particular application of the invention mentioned above, i.e. repairing a tube in the steam generator of a nuclear reactor, and in order to remedy the drawbacks enumerated above, the present invention provides a method enabling a bush to be welded in a steam generator tube by means of a source of energy which is situated at a very considerable distance from the steam generator, e.g. outside the reactor building in which it is contained. The invention also provides apparatus for implementing the method.
The method makes it possible to perform the following operations automatically and under remote control from a control station situated, for example, outside the reactor building:
an optical weld head is inserted into the tube to be repaired after a bush has been fitted therein by a conventional method, said head is connected to a laser (e.g. of the YAG type) situated close to the control station, said connection being provided by a flexible tube containing an optical fiber for transporting light energy, an inert gas feed duct, and cables for transmitting control signals;
the weld head is positioned at a working level;
the focusing distance of the beam is adjusted; and
a full circumference of the bush is welded in each of two zones on either side of the leaky zone of the tube.